Terahertz Technology

Terahertz Technology

THz radiation

Terahertz (THz) Technology is a highly topical field of research. The THz-frequency domain has become more easily accessible during the last 20 years due to the development of efficient emitters and detectors, strongly coupled with the availability of powerful femtosecond-lasers. THz-radiation features some very attractive properties that open a broad field of applications.

Abb.: Terahertz-Frequency domain [Ref: Tatoute]

Interesting physical properties of THz-radiation

  • THz-radiation is located between the infrared and the microwaves in the electromagnetic spectrum, whereas 1 THz (=10^12Hz) corresponds to a wavelength of 300 µm. THz-radiation penetrates many materials (an advantage compared to infrared) and also exhibits a better spatial resolution than microwaves.
  • The penetration depth of THz-waves is relatively large for many non-conductive materials, such as plastics, paper, cardboard, textiles, etc. This allows for the use of THz-radiation for investigating the inner structure of samples in search of defects, inclusions, etc. by THz-imaging.
  • Several substances show characteristic fingerprints at THz-frequencies due to collective molecular excitations. This allows the identification and characterization of substances such as pharmaceuticals, drugs, explosives, etc. by THz-spectroscopy.
  • THz-radiation exhibits very low energies (1 THz corresponds to an energy of approximately 4meV, in comparison to room temperature at approximately 25 meV) and is consequently non-ionizing.

Measuring principles of THz-time-domain spectroscopy

A frequently used method is THz-time-domain spectroscopy (THz-TDS). This method can be used for imaging as well as for spectroscopy and employs pulsed THz-radiation. As the THz-pulses have durations of only a few picoseconds, a direct electronic measurement of the pulses is not possible to-date. Therefore, the THz-pulses are sampled in a time-resolved wave. The most important parts of a standard THz-TDS setup are as follows:

Schematic setup of a THz-TDS-System(top), time resolved THz-pulse (bottom right) and THz-spectrum computed by FFT (bottom left).

  • A femtosecond laser emits very short laser pulses, which only last a few tens of femtoseconds (femto = a millionth of a billionth). The fs-laser pulses are divided by a beam splitter and are sent to a THz-emitter and THz-detector, where they excite THz-radiation or are used for detecting the THz-pulse. In our case, the THz-emitter and THz-detector are photoconductive antennas, which consist of semiconductor material comprising a metallic antenna structure.
  • A mechanical delay line introduces a variable time delay between excitation and detection of the THz-pulse, enabling time resolved sampling.
  • The signal from the THz-detector needs to be amplified (using a current amplifier and a lock-in-amplifier), so that the very small THz-pulse can be digitized and displayed on a computer.
  • The THz-pulse is transformed into a THz-spectrum by Fourier transformation (Fast Fourier transformation; FFT). The short duration of the pulse leads to a broad THz-spectrum, which can be used directly for spectroscopy.

Works at RECENDT GmbH

The “THz Technology” research area at RECENDT GmbH focuses on methods for THz-spectroscopy (time-domain spectroscopy; TDS) and THz-imaging for the investigation and characterization of materials within the following topics:

  • THz-Technology for industrial problems
  • Investigation and development of scanning THz-Systems
  • Polarization-sensitive (PS) maesurements for the analysis of orientations
  • High-speed and spatially resolved THz spectroscopy

Currently, there are three THz-TDS laboratory setups with the following properties available: Spectroscopy:


Spectroscopy:

  • Measurements in transmission and reflection
  • Nitrogen purging
  • Point-wise and spatially resolved THz Spectroscopy


Imaging:

  • Measurements in transmission and reflection
  • Special scanning THz system for industrial samples
  • PS-THz for the investigation of orientations

Examples of applications

Quality control: detecting defects in a sheet of glue

Ref: S. Katletz, M. Pfleger, H. Pühringer, N. Vieweg, B. Scherger, B. Heinen, M. Koch, and K. Wiesauer, „Efficient Terahertz En-face Imaging“, Opt. Express 19, 23042–23053 (2011)

Inner structures: “inner life” of a toy figure

Hyper-spectral imaging (HSI): space-resolved spectroscopy on lateral inhomogeneous samples

Ref: H. Pühringer, J.Kasberger, M. Pawliczek, S. Katletz, M. Pfleger, H. Lohninger, and K. Wiesauer, "Terahertz Hyperspectral Imaging: Comparison of Different Evaluation Methods", International Forum on Terahertz Spectroscopy and Imaging, 06.-07.03.2012, Kaiserslautern (Germany)

Industrial Applications

Tribological coatings

Metallic components are often coated dielectrically in order to optimize the operating characteristics (such as tribology).
With Terahertz technology (THz) such coatings can be measured efficiently inline.

Hidden (adhesive-) layers

With the Terahertz technology (THz) it is possible to look through relatively thick plastic layers and test hidden layers non-destructively (e.g. adhesive layers). The terahertz waves used are not harmful for health (no ionizing radiation), but still make it possible to look inside many optically non-transparent materials.

Spectroscopic imaging

The spectroscopic control of chemical compositions (e.g. of pills) can be achieved by implementing different technologies (NIR, MIR, Raman, and THz). Spatially resolved spectroscopy is also possible – moreover you can get a picture, for instance, of the API-distribution in the product!

Spectroscopy in the product development, production and QA

For the production monitoring / quality control or the product development, spectroscopic technologies (NIR, MIR, Raman, THz) can play a significant role. These methods make it possible to obtain very precise measurements of mixing ratios, compositions, ingredients, reaction progresses, and similar chemical information without sample preparation and costly laboratory analyses.
Such real-time measured data provides an ideal basis for your process optimization!

Multilayer pipe extrusion (Co-extrusion)

With THz technology, individual layer thicknesses can be measured inline in the multilayer pipe extrusion process (co-extrusion). This allows a precise regulation and optimization, with savings in energy and raw material, which does not hinder quality assurance.

Fiber directions in composite materials

Do you want to gain a deeper insight and understanding of interior properties of your fiber-reinforced plastics? Are you interested in the anisotropies of your components or do you need to determine the orientation of the fibers inside a CFRP-injection moulded component?
We can help with our Terahertz-Technology (THz), OCT technology, and Laser Ultrasonics Technology.

Explosives or drugs identification

Spectroscopic methods allow the easy identification of substances. Using such methods makes it possible to identify explosives and to distinguish between non-hazardous materials and hazardous or illegal substances.

Moreover, with Terahertz Spectroscopy, such analysis can also be performed for substances inside closed and non-transparent containers / objects (e.g. letters and cardboard boxes).

Security checks

Terahertz imaging received significant media attention in connection with the so called “body scanners” for airport security checks. Indeed, it is possible to detect THz radiation, which is emitted from every body, through clothing using the THz technique. These images can help to detect shadows, such as hidden weapons.

Analytics for paper & the paper industry

Paper is an extremely diverse material and its characterization requirements are relevant for production, processing, and application. We can support you in many ways with our testing methods (IR & Raman Spectroscopy, THz Spectroscopy, OCT, and Laser-Ultrasound), for example, in chemical analytics (directly in the production process) and the characterization of surfaces and fibers for both absorbency and anisotropy.